The present invention relates to a process for producing hard magnetic material, and more particularly relates to a process for producing so-called anisotropic fine grain type hard magnetic material in which fine grains each corresponding to a unit magnetic domain are dispersed with shape anisotropy into a nonmagnetic base.
It is well known to public to produce hard magnetic material with magnetic anisotropy and high magnetic characteristics by dispersing with prescribed orientation into a base of nonmagnetic substance such as Cu, Al and Sn anisotropic fine grains of highly magnetic or ferromagnetic substance such as Fe, Co, Ni and Fe-Co alloys, each grain corresponding to a unit magnetic domain.
In one actual process for production of such hard magnetic material, cast Fe-Ni-Al-Co type alloy, or like alloy further including Cu, Ti and/or Nb, is thermally treated within magnetic field in order to cause so-called spinodal decomposition which eventuates in dispersed separation of highly magnetic, fine grains with shape anisotropy within a nonmagnetic phase. This process, however, results in high material cost due to use of costy metals such as Co and Ni. In addition, the thermal treatment within magnetic field requires use of an exorbitant equipment, and causes high process cost and low productivity. Further, the hard magnetic material produced by this process is too hard and fragile to be worked and/or cut smoothly.
In another actual process for production of the above-described hard magnetic material, fine, spherical Fe grains, each having a diameter in a range from 15 to 30 mm and corresponding to a unit magnetic domain, are obtained by a reduction process. Such Fe grains are then blended with grains of nonmagnetic metal such as Al, and the resultant blend is subsequently subjected to compaction and sintering.
In the case of this process, however, it is extremely infeasible to prepare fine grains of uniform diameter, each corresponding to a unit magnetic domain, since such fine grains easily aggregate even in a free state due to mutual magnetic attraction. Relatively large specific surface area of each fine grain tends to cause oxidization of the grains, which disables easy and simple handling of the substance. Oxidization of the substance seriously degrades saturated magnetic flux density Bs and 4.pi. Is of the resultant, sintered material. Such oxidization further impairs affinity of the grains with the grains of nonmagnetic substance, thereby seriously deteriorating mechanical strength of the obtained sintered body. I11 spinal rotation due to presence of the spherical grains is liable to connect to unstable magnetic characteristics of the product.
In the other actual process of the above-described hard magnetic material, long and thin Fe fine grains or Fe-Co alloy fine grains are separated on Hg electrodes by electrolysis which are then dispersed into nonmagnetic substance, the blend is processed to compaction for orientation of the Fe fine grains, and the compacted body is subjected to sintering. Although this process significantly improves magnetic characteristics of the product, it is still very difficult to obtain, at high efficiency, fine grains of uniform dimension. Further, this process, just like the foregoing instance, cannot avoid the oxidization troubles.
It is also proposed in actual production of the hard magnetic material of the above-described type to subject a highly magnetic core rod, e.g. an Fe rod, covered with a nonmagnetic sheath, e.g. an Al sheath, to repeated drawings for plastic deformation, which microminiaturizes and disintegrates the highly magnetic substance into mutually separated fine grains, each corresponding to a unit magnetic domain, dispersed within the nonmagnetic base.
In this case, however, high frictional contact between the highly magnetic core covered with the nonmagnetic sheath and the dies disables uniform flow of the substances during the process. Consequently, the highly magnetic fine grains are oriented substantially in parallel to the axial direction of the product in its central section whereas they are arranged at random in the peripheral section of the product. Such untidy arrangement of the highly magnetic fine grains within the obtained structure seriously deteriorates magnetic characteristics of the resultant hard magnetic material. Since it is difficult to design a high rate of cross-sectional reduction for each drawing, drawings have to be repeated several times in order to obtain a product of a desired diameter, thereby considerably raising production cost.
A process for solving such problems has already been proposed by inventors of the present invention in Japanese Publication Sho. No. 51-21947, in which the conventional drawing process is replaced by hydrostatic extrusion process for production of a hard magnetic material. This proposal is based on a recognition that relatively low frictional contact between the work piece and the die allows smooth and tidy flow of the substances and relatively high rate of cross-sectional reduction is employable in the case of the hydrostatic extrusion. In this proposed process, a plurality of elongated highly magnetic cores each covered by nonmagnetic sheath are bundled together and subjected to hydrostatic extrusion for plastic deformation. As a result of such plastic deformation, highly magnetic fine grains, each corresponding to a unit magnetic domain, are oriented and dispersed within the nonmagnetic base so that their longitudinal directions substantially meet the axial direction of the produced hard magnetic material which has a composit structure with shape anisotropy.
This process assures ideal orientation of the fine grains, each corresponding to a unit magnetic domain, and, consequently, greatly improved magnetic characteristics of the product. Production requires reduced repetition of the unit operation, i.e. the hydrostatic extrusions, thereby remarkably lowering the production cost.
Further study of this previous process by the inventors, however, has revealed presence of the following disadvantage. As described already, the highly magnetic core, e.g. an Fe rod, is covered with the nonmagnetic sheath such as Al covering in the case of this previously proposed process, prior to the hydrostatic extrusion. More specifically, a Fe rod is inserted into a small/Al cylinder, whose inner wall is covered with Al.sub.2 O.sub.3 layer, in order to form a composite body. A plurality of such composite bodies are bundled together, inserted into a large Al cylinder and subjected to hydrostatic extrusion for cross-sectional reduction of the composite bodies.
Since each Fe rod is broken into fine pieces during the plastic deformation, cross-sectional reduction tends to vary from piece to piece. Consequently, the Al base in the product contains Fe fine grains of different diameters. Some fine grains may be larger in size than the unit magnetic domain, and uncontrollable presence of such large fine grains dispersed in the base leads to unstable magnetic characteristics of the obtained hard magnetic material. With this previous process, it is almost infeasible to control the hydrostatic extruction so that the product should contain Fe fine grains only which correspond in size to the unit magnetic domain. When compared to the conventional production by drawing, use of hydrostatic extrusion remarkably reduces repetition of the unit operation thanks to its relatively large extrusion ratio. Yet, appreciable repetition of the hydrostatic extrusion is necessary to microminiaturize the starting rod to the fine grains each corresponding to a unit magnetic domain. Employment of high rate cross-sectional reduction for hydrostatic extrusion in this process may limit free choice of the substances to be used due to expected high resistance against plastic deformation.